1. Field of the Invention
The present invention relates generally to an image forming apparatus, and more particularly to an image forming apparatus that has a developing-device assembly, which is rotated to switch a plurality of developing devices, and forms an image based on electrophotography.
2. Description of the Related Art
An image forming apparatus that forms an image based on electrophotography includes a photosensitive member, a charging device, an exposing device, a developing device, a transfer device and a fixing device. An image forming apparatus for monochromatic printing has one developing device, whereas an image forming apparatus for multi-color printing includes a plurality of developing devices and may additionally include an intermediate transfer member.
The image forming apparatus for multi-color printing includes a developing-device assembly comprising a plurality of developing devices. The developing-device assembly is rotated like a cylinder of a revolver-pistol, whereby each developing device is switched. Each developing device includes a developing roller and a toner container. The toner container contains toner to be supplied to the developing roller.
In general, the multi-color printing image forming apparatus forms a multi-color image using three colors, yellow, magenta and cyan, or four colors, i.e. these colors plus black. If the toner of any one of these colors runs short, a multi-color image of a desired color tone cannot be obtained. It is thus necessary to understand the residual amount of toner in each developing device.
The residual toner amount in the toner container cannot be confirmed from the outside of the image forming apparatus, so a sensor for detecting the residual toner amount is provided for each toner container. In this case, since the toner container rotates along with the developing-device assembly, such a sensor cannot be attached by wiring. To cope with the problem, a transparent window is provided at a part of the toner container, and the residual toner amount is optically detected through the window.
In a case where toner adheres to the inside of the window, however, the residual toner amount may not correctly be detected. In order to enhance the reliability in detection of the residual toner amount, it is desired to directly provide the sensor on the toner container. However, since the developing-device assembly rotates, electric power cannot be supplied to the sensor.
Jpn. Pat. Appln. KOKAI Publication No. 5-66681 discloses an image forming apparatus that uses a method of supplying power to a rotary member. According to this publication, a generator is provided between a rotary photosensitive drum and a shaft that supports the photosensitive drum, and the generator supplies electric power to a heater built in the photosensitive drum. In this case, the generator generates power when the photosensitive drum rotates. In addition, a battery is provided to enable power supply even when the photosensitive drum is at rest.
In the case where this method is applied to supply electric power to devices mounted in the developing-device assembly, the following problem arises. Since the developing-device assembly rotates only intermittently, power is supplied only when the developing-device assembly rotates. Further, the number of revolutions of the developing-device assembly, compared to that of the photosensitive drum, is not sufficient for power generation or power accumulation.